1. Field of the Invention
The present invention relates to a reflective liquid crystal display device comprising a reflective pixel electrode, a method of manufacturing the same, and a liquid crystal display unit such as a reflective liquid crystal projector which displays an image through the use of the reflective liquid crystal display device.
2. Description of the Related Art
In recent years, with improvement in definition, miniaturization and brightness of projection displays, as display devices of the projection displays, reflective devices capable of reducing their size and displaying with high definition, and being expected to have high light utilization efficiency have become a focus of attention and have been put to practical use. A well-known reflective device is an active type reflective liquid crystal device in which a liquid crystal is injected between a pair of substrates facing each other. In this case, as the pair of substrates, a transparent electrode substrate formed through laminating a transparent electrode on a glass substrate, and a drive substrate using a silicon (Si) substrate which includes, for example, a CMOS (Complementary-Metal Oxide Semiconductor) type semiconductor circuit are used. On the drive substrate, a reflective pixel electrode of metal for reflecting light and applying a voltage to the liquid crystal is disposed so as to form a pixel electrode substrate. The reflective pixel electrode is made of a metal material including aluminum as a main component, which is generally used in a LSI (Large Scale Integrated) process.
In such a reflective liquid crystal display device, when a voltage is applied to the transparent electrode disposed on the transparent electrode substrate and the reflective pixel electrode disposed on the drive substrate, a voltage is applied to the liquid crystal. At this time, a change in optical properties of the liquid crystal occurs depending upon a potential difference between these electrodes, thereby the liquid crystal modulates incident light. The reflective liquid crystal display device can display a gray-scale image by the modulation of the light.
In such a reflective liquid crystal display device, specifically an active type reflective liquid crystal display device into which a vertically aligned liquid crystal is injected has become a focus of attention as a projection device in recent years, because the active type reflective liquid crystal display device has high contrast and high response speed. Herein, “a vertically aligned liquid crystal material” means a liquid crystal material with negative dielectric anisotropy (a difference Δ∈(=∈(∥)−∈(⊥)) between a dielectric constant ∈(∥) parallel to a long axis of liquid crystal molecules and a dielectric constant ∈(⊥) perpendicular to the long axis of the liquid crystal molecules is negative), and in the vertically aligned liquid crystal material, when an applied voltage is zero, the liquid crystal molecules are aligned in a substantially vertical direction with respect to a substrate surface, thereby the active type reflective liquid crystal display device operates in a normally black mode.
In the vertically aligned liquid crystal, when an applied voltage is zero, the long axis of the molecules of the vertically aligned liquid crystal is aligned in a substantially vertical direction with respect to each substrate surface, and when a voltage is applied, the long axis is inclined in an in-plane direction, thereby the transmittance of the vertically aligned liquid crystal changes. If the directions where the liquid crystal molecules are inclined are not uniform during drive, the contrast becomes uneven. In order to prevent uneven contrast, it is required to align liquid crystal molecules at a very small pretilt angle in a predetermined direction in advance, then vertically align them. The predetermined direction is a diagonal direction of the pixel electrode (that is, a 45° direction). When the pretilt angle is too large, vertical alignment is degraded, thereby a black level is increased, and the contrast declines. Therefore, in general, the pretilt angle is controlled within a range from approximately 1° to 5° with respect to the direction of the normal to a substrate surface.
There are two methods of aligning the vertically aligned liquid crystal material, that is, a method of controlling alignment by using an organic alignment film typified by polyimide and rubbing it, and a method of controlling alignment through oblique evaporation by using an inorganic alignment film typified by silicon oxide. Nowadays, in order to achieve higher brightness of a projector, there is a tendency that the power of a lamp is increased to irradiate a display panel with light with very high intensity. Therefore, there is a problem that the organic alignment film in the former method is degraded due to the light.
On the other hand, an obliquely evaporated film of silicon oxide in the latter method is an inorganic material, so unlike polyimide, no degradation in the material due to light occurs, and higher reliability can be achieved. Therefore, the obliquely evaporated film becomes a focus of attention. In the case where the alignment film is the obliquely evaporated film of silicon oxide, the incident angle of evaporation particles to a substrate is changed during oblique evaporation to control the pretilt angle. In general, a practical incident angle is within a range from approximately 45° to 65° with respect to the direction of the normal to a substrate.
Techniques of the related art regarding the reflective liquid crystal display device are proposed in, for example, Japanese Unexamined Patent Application Publication Nos. Hei 11-174427 and 2001-5003.
At first, a first problem with the techniques of the related art will be described below. FIG. 1 shows a cross sectional photograph of a film structure of the transparent electrode substrate as an example of the structure of an alignment film by a scanning electron microscope. In the film structure, an ITO (Indium Tin Oxide) film as a transparent electrode is formed on a glass substrate, and a silicon oxide (SiO2) film as an alignment film is formed directly on the ITO film through oblique evaporation. It is obvious from the cross sectional photograph that the silicon oxide film formed through oblique evaporation has a columnar structure which is inclined to the evaporation direction. Although it is considered that a vertical liquid crystal can be inclined at a pretilt angle by such a structure, as can be seen from the cross sectional photograph, in the structure, a large number of gaps exist, so the silicon oxide film is not exactly a dense film. Therefore, ions generated from an electrode during drive of a liquid crystal cell, ions existing in a liquid crystal cell, or ions or impurities generated in a liquid crystal cell by light easily pass through the silicon oxide film, so the silicon oxide film is a film with a relatively low resistance. The same goes for an alignment film on a pixel electrode substrate side.
Therefore, in the case where a liquid crystal cell is driven for a long time, ions are introduced into the liquid crystal cell, and the deviation of ions occurs in the liquid crystal cell, thereby resulting in so-called burn-in. In order to correct the deviation of ions, a technique of changing a thickness ratio between the alignment films on both sides of the liquid crystal or a technique of inserting a different dielectric layer so as to electrically correct electrical asymmetry generated by the deviation of ions, thereby reducing the deviation of ions can be considered. However, burn-in occurs because the alignment films formed through oblique evaporation are not dense, so the former technique produces little effect. Further, the latter technique is not practical, because a problem that new burn-in occurs in an interface between the dielectric layer and the alignment film may arise, or there are problems such as the need for forming a film made of another material in manufacturing.
In Japanese Unexamined Patent Application Publication No. Hei 11-174427, a device with a structure in which a layer made of a material different from that of an alignment film is formed between an electrode and the alignment film is described. In Japanese Unexamined Patent Application Publication No. Hei 11-174427, as a structure for preventing light reflection which occurs in an interface between a liquid crystal and a glass substrate, a technique regarding a liquid crystal display device with a laminate structure in which a transparent electrode layer, an alignment layer and one or a plurality of transparent middle layers with a refractive index which is lower than that of the transparent electrode layer and is higher than that of the liquid crystal layer or a glass substrate are laminated on an inner surface of the glass substrate is disclosed. As a specific example, a structure in which an Al2O3 film as a middle layer is formed on an ITO electrode film, and an obliquely evaporated film of SiO2 as an alignment film is formed on the Al2O3 film is described.
However, the technique in Japanese Unexamined Patent Application Publication No. Hei 11-174427 is a technique for preventing light reflection from an optical standpoint, so the technique does not solve the problems such as passage of ions which are concerned with the invention. Moreover, the problems such as passage of ions occur in not only a transparent electrode substrate but also a pixel electrode substrate; however, the technique in Japanese Unexamined Patent Application Publication No. Hei 11-174427 is only for the transparent electrode substrate.
Next, a second problem with the techniques of the related art will be described below. In general, it is difficult to control the alignment of a vertically aligned liquid crystal material. In the case where there is an uneven structure on the drive substrate by the reflective pixel electrode or the case where a groove exists between pixel electrodes, an alignment defect occurs around the pixel electrode due to the uneven structure. The alignment defect causes a decline in uniformity of characteristics in a display surface, an increase in black level (a phenomenon in which a black portion of an image does not appear black but gray), degradation in image quality due to disclination. In particular, in a reflective liquid crystal display device using a silicon drive device, a pixel pitch is as small as 10 μm or less in general, so compared to a large direct-view-type liquid crystal device with a pixel pitch of a few tens of μm or more, a defective region around a pixel easily exerts an influence upon image quality, and unlike a transmissive liquid crystal display device, the defective region cannot be covered with a black matrix, so a basic practical requirement for the reflective liquid crystal display device is that a misalignment region must be minimized or completely eliminated.
A problem of a reflective liquid crystal display device due to the structure of a pixel electrode will be described in detail below. As shown in FIGS. 2A and 2B, reflective pixel electrodes 111 are arranged on a silicon drive substrate 110 in a matrix. The size and the shape of each of the reflective pixel electrodes 111 is, for example, a square 8.4 μm on a side. In order to prevent an electrical short circuit between adjacent pixels, the reflective pixel electrodes 111 are disposed so as to have an inter-pixel space W1 with a predetermined distance. When the inter-pixel space W1 is, for example, 0.6 μm, a pixel pitch W2 is 9 μm. In general, the pixel pitch W2 is within a range of approximately 7 μm to 15 μm, and the inter-pixel space W1 is within a range of approximately 0.3 μm to 0.7 μm. Further, the thickness of the pixel electrode is within a range of approximately 150 nm to 250 nm.
As each of the reflective pixel electrodes 111 has such a shape, a portion with a groove-like shape (hereinafter referred to as inter-pixel groove) is always formed between adjacent pixel electrodes. The inter-pixel groove has, for example, an aspect ratio of 150 nm high to 600 nm wide in a sectional surface shown in FIG. 2B.
FIGS. 3 and 4 schematically show a state where an alignment film 112 of silicon oxide is formed on a pixel structure shown in FIGS. 2A and 2B through oblique evaporation and a state of the alignment of a vertically aligned liquid crystal 113 by the alignment film 112. In FIGS. 3 and 4, an arrow 130 shows an evaporation direction where the alignment film 112 is formed. The alignment film 112 is obliquely evaporated on a substrate at, for example, an incident angle θ of 55° (refer to FIG. 3) with respect to the direction of the normal to a substrate surface from a diagonal direction of the reflective pixel electrodes 111 (refer to FIG. 4).
In the case where such oblique evaporation is performed, as shown in FIG. 3, an area around a side surface of the reflective pixel electrode 111 opposite to the incident direction (around a region 121 in FIG. 3) is shaded with the reflective pixel electrode 111, so the alignment film 112 is not evaporated and formed on the area. On the other hand, the alignment film 112 is formed in the shape of the letter L around a side surface on the other side, as shown in FIG. 3. Thus, the region 121 where no alignment film 112 is formed exists on a bottom surface of the inter-pixel groove and the side surface of the reflective pixel electrode 111.
The alignment direction of a pretilt is a diagonal direction of pixels, and FIG. 4 shows a schematic plan view of a region where the alignment film 112 is formed and the region 121 where the alignment film 112 is not formed in this case. When the thickness of the reflective pixel electrode 111 is increased, and the inter-pixel space W1 is reduced, no film is evaporated on the bottom surface of the inter-pixel groove, and the film is formed only on one side surface of the inter-pixel groove. In a typical method of forming an alignment film, it is inevitable that the film structures of both side surfaces of the inter-pixel groove become asymmetric.
Thus, as a region where the alignment film 112 is not formed specifically on the bottom surface of the inter-pixel groove exists, the alignment of the liquid crystal 113 cannot be controlled in the region, thereby the liquid crystal 113 goes out of alignment, and problems such as degradation in image quality such as nonuniform alignment and a decline in reliability arise. In other words, as shown in FIG. 3, the alignment film 112 is formed on a surface of the reflective pixel electrode 111, thereby the long axis of liquid crystal molecules is uniformly aligned in a pretilt direction in a good state in general. On the other hand, specifically the region 121 where the alignment film 112 is not formed is produced in a portion of the bottom surface of the inter-pixel groove, so a force to vertically align the liquid crystal molecules does not work, thereby a nonuniform alignment region 120 is produced. The nonuniform alignment region 120 exerts an influence upon an area around the pixel electrode, thereby resulting in a state where liquid crystal molecules on the surface of the pixel electrode are vertically aligned, but liquid crystal molecules in a region from an area around the pixel electrode to the inter-pixel groove are nonuniformly aligned. Thereby, nonuniform alignment occurs in the region from the area around the pixel electrode to the inter-pixel groove, so degradation in image quality is induced. The evaporation angle is generally selected within a range of 45° to 65° with respect to the direction of the normal to the substrate; however, the deeper the inter-pixel groove is, the larger the region where the alignment film 112 is not formed in the bottom surface of the inter-pixel groove becomes, so the evaporation angle has a large influence. The above phenomenon specifically occurs in the case where an obliquely evaporated film of an inorganic material such as silicon oxide is used as the alignment film 112.
On the other hand, in an organic alignment film such as polyimide, the above problems which arise because the alignment film 112 is not formed do not occur. It is because the organic alignment film is formed through coating the whole surface of the pixel substrate with a material in a solvent form by a technique such as spin coating, so the inter-pixel groove is overcoated with the material on average.
As a method for preventing such problems with the obliquely evaporated film, in Japanese Unexamined Patent Application Publication No. 2001-5003, a method in which at first, oblique evaporation is performed at an angle of 70° from the normal to a substrate surface along a side of a pixel electrode to form a first alignment film on a bottom surface portion (refer to as A) of an inter-pixel groove along the side of the pixel electrode, and then the substrate is rotated by 90° in a plane, and a second alignment film is formed on a bottom surface portion (refer to as B) of the inter-pixel groove along the other side of the pixel electrode through the same oblique evaporation is proposed.
The technique described in Japanese Unexamined Patent Application Publication No. 2001-5003 is a technique regarding a method of separately forming the first alignment film and the second alignment film through oblique evaporation from different incident direction in a substrate surface and an in-plane substrate rotation system for changing the evaporation direction in a plane. By the technique, an alignment film is surely formed on the bottom surface portion of the inter-pixel groove. However, the first alignment film and the second alignment film are formed from different directions (90° different in an example) in a plane, so the pretilt direction of the liquid crystal is different in the bottom surface portions A and B, so when a voltage is applied, liquid crystal molecules are inclined to different directions.
In other words, the second alignment film is formed on the surface of the pixel electrode, so most of liquid crystal molecules including liquid crystal molecules in a pixel groove portion B are inclined to a direction determined by the second alignment film. However, liquid crystal molecules in a pixel groove portion A on which the first alignment film is formed are inclined to a direction different from the above direction, so in a portion between them, that is, an area around the pixel groove portion A, a portion in which the alignment direction is locally different from an area around the portion is produced. The portion is very small, but it looks a periodic unevenness in alignment. Moreover, in the technique, as described above, the alignment film cannot be formed on the whole inter-pixel groove to be exact unless evaporation is performed along a side of the pixel electrode, and the substrate is rotated by 90° in a plane to perform evaporation again.
In general, in the reflective liquid crystal display device, as a polarization splitting device, a PBS (Polarization Beam Splitter) is used. When polarization is split in a cross Nicol arrangement by the PBS, the alignment direction of a vertical liquid crystal which can obtain the highest transmittance is diagonal to a pixel, that is, a 45° direction. Therefore, in the alignment along the side of the pixel in Japanese Unexamined Patent Application Publication No. 2001-5000, a polarization splitting optical system using the PBS cannot be used in the reflective liquid crystal display device, and the reflective liquid crystal display device has little practicability specifically as a projection display unit. In order to avoid the problem, when the second alignment film is formed in a direction diagonal to the pixel, in principle, even if the first alignment film is formed from any direction, a region which cannot be fully covered exists in the inter-pixel groove, so effects of the technique in Japanese Unexamined Patent Application Publication No. 2001-5003 are not exerted. Therefore, the technique is far from practically effective.